Salt and Water Transport and its control Flashcards
Electrolyte and water transport in the GI tract
Water and electrolytes provide medium for
o Digestive processes within the GI tract
o Metabolic processes within the body on absorption
o Replace daily loss of body fluids in sweat, urine, lungs and faeces
When regulatory processes fail it can lead to life threatening diarrhoea and electrolyte imbalance
Tight regulation of water and electrolytes
Net absorption and minimal loss of water and electrolytes in faeces occurs (~200mL).
Secretion/absorption regulated by
o gut luminal contents (osmolarity)- in absence of food electrolytes are primarily responsible for creation of osmotic pressure in gut lumen
o enteric and autonomic signals
o endocrine hormones
o immunogenic signals
Efficient conservation of water and salts
Bidirectional secretion and absorption occurs across GI epithelium daily
Secretions dominate in upper GI tract (~7L): saliva, bile, gastric and pancreatic juice, intestinal juice
o Absorption dominates overall: 98% (~8.8 L) of ingested and secreted water and electrolytes are absorbed
o Facilitate movement along the GI tract, mixing with digestive enzymes, chemical reactions, nutrient absorption
Primary site for absorption - small intestine
Ingested and secreted water and electrolytes are predominantly absorbed in the small intestine (jejunum)
Distinct secretion and absorptive profiles depend on variations in epithelial membrane transport proteins and permeability along the GI tract
Small intestine adaptations
Surface area: folds, villi, microvilli
Villi lymphatic and blood vessels
Enterocyte actin microfilaments rhythmically contract to move microvilli for maximum exposure to lumen contents
Rapid response to chyme
Hypertonic: osmosis of water into lumen to form isotonic chyme
Acidic: rapid increase in HCO3- -rich secretions
Epithelium is more permeable than the large intestine
Large Intestine adaptations
No villi but surface is covered with crypts/ intestinal glands
Smaller role in transport of water and salts, bacterial microbiome role in protein digestion/ vitamin synthesis
Smaller role in digestion: nutrient adsorption is limited (digestive enzyme activity absent)
Epithelium is less permeable than small intestine
Has additional absorptive capacity for water and NaCl in exchange for K+ loss (4-6 L)
Electrolytes
Electrolytes dissociate in solution into ions – carry an electrical charge
The concentration of different electrolytes differs in the cytosol and extracellular fluid
BUT osmotic balance is maintained
An electrochemical gradient occurs across the cell membrane
Ions will diffuse down their electrochemical gradient if mechanisms are present to do so
Primary and Secondary Active Transport
Primary active transport: Primary active transport; Hydrolysis of ATP provides energy to move ions against their electrochemical gradient; Na+-K+ ATPase pump
Secondary Active transport: Uses gradient created by primary pump to move substance against it’s electrochemical gradient; Symport = two molecules in same direction; Antiport = two molecules in opposite direction
Sodium potassium ATPase pump
3 Na+ exported and 2 K+ imported against electrochemical gradient using energy from ATP hydrolysis by ATPase
Keeps Na+ cytosol concentration low
Important mechanism driving gut absorption
Symport and antiport transport mechanism
Secondary active transport using the gradient supplied by Na+-K+ ATPase active transport of Na+ out of the cell.
Na+ is then transported down it’s concentration gradient into the cell with (symport) or in exchange for (antiport) other molecules, moving them against their concentration gradient
Principles of enterocyte transport
Polarised with an apical and basolateral membrane
Tight junctions provide a barrier to free flow of gut lumen contents
Tight junctions more permeable in proximal small intestine
Tonicity of chyme entering duodenum affects bidirectional fluid flux
Occurs by transcellular and paracellular routes
Transcellular absorption may be against concentration gradient and require ATP
Paracellular routes (between cells) do not require energy
Types of electrolyte transport
Passive: down electrochemical gradient through ion channel or carriers or permeable tight junctions
Solvent drag: water follows Na+ gradient via osmosis, taking other ions (e.g. upper intestine where tight junctions are more permeable)
Active: requiring ATP, Na+/K+ ATPase pump depletes cellular Na+ and draws Na+ across apical membrane from gut lumen via channel or cotransporter
Key variations in water and electrolyte transport in the small intestine
Chyme contains water and key electrolytes Na+, K+, Cl, and HCO3- from ingested food and secretions of the GI tract
Rapid osmotic equilibration in the duodenum to form isotonic chyme
o secretion of H2O into hypertonic chyme
o absorption of H2O from hypotonic chyme
Jejunum absorbs Na+, K+, Cl- and H2O and Ileum secretes HCO3- (6.5L absorbed).
Na+ and Cl- are conserved, HCO3- secretion
Key variations in water and electrolyte transport in the large intestine
Smaller volume of chyme (~2 L) enters large intestine, 100-200 ml is excreted
Water and electrolytes are absorbed in proximal colon
Tight junctions prevent back-diffusion of ions into lumen allowing more complete NaCl absorption, limited loss in faeces
Secretion of HCO3- and K+ occurs
Na+ absorption in the small intestine
Drives absorption of ions, organics, H2O
Driven by basolateral active transport of Na+ into the interstitial space via Na+/K+ ATPase pump
Dominance of mechanisms vary along the small intestine:
o Via passive diffusion
o Via apical membrane carrier proteins
Organic substrate Na/glucose and Na/amino acid co-transporters
Na/H antiport
Na/Cl symport (ileum)
